Remove top-level BUILD file (#54)

Given that the top-level `BUILD` file is otherwise empty, the bug
that is obliquely referenced here is likely to be
thought-machine/please#3408, which prevents
repos from being properly initialised unless they have a top-level
`BUILD` file. This bug was fixed in Please v17.19.1, so require that as
the minimum version of Please for using the plugin.

Author: chrisnovakovic

.plzconfig

@@ -1,5 +1,5 @@
 [Please]
-version = >=17.10.1
+version = >=17.19.1
 
 [Plugin "java"]
 Toolchain = //third_party/java:toolchain